The present invention relates to a gas component detection apparatus for detecting the variation in concentrations of gaseous components such as oxygen (O.sub.2), carbon monoxide (CO) and hydrocarbon (HC) of for example exhaust gases from an internal combustion engine.
Gas component detection apparatuses have been widely used in many industrial fields. Lately, as a countermeasure to cope with the problem of exhaust gases from an internal combustion engine, gas component detection apparatuses have been employed for determining the air-fuel ratio of an air-fuel mixture supplied to the internal combustion engine.
In the case where a catalyst is utilized for purifying exhaust gases from an internal combustion engine, the catalyst cannot exhibit maximum properties unless the air-fuel ratio of an air-fuel mixture is maintained constantly at a proper value. However, in an ordinary internal combustion engine equipped with a conventional carburetor or a fuel injection apparatus, the air-fuel ratio is actually inevitably subjected to a large variation even when the ratio of an injected fuel to intake air is set to be constant. Consequently, in order to maintain constantly a proper air-fuel ratio, it is necessary to detect with the use of gas detection apparatus the air-fuel ratio prior to burning of the air-fuel mixture and feed back a signal corresponding to the detected value to the carburetor or the injection apparatus, thereby controlling the air-fuel ratio of the air-fuel mixture supplied to the engine.
Gas component detection apparatuses are constructed to determine the air-fuel ratio based on the fact that the variation in concentrations of gaseous components of the exhaust gases is closely related to variation of the air-fuel ratio of the air-fuel mixture. In this connection, consideration has to be given to the fact that the temperature of the exhaust gases, as well as the concentrations of the gaseous components thereof, will vary abruptly and remarkably. It is thus desirable that the gas component detection apparatuses be operable with high accuracy notwithstanding such prominent variables.
Heretofore, a gas component detection apparatus has been known which employs transition metal oxide. In the case where the air-fuel ratio of an air-fuel mixture is determined by employing such detection apparatus, a differential operational amplifier which has a non-inverted input terminal and an inverted input terminal is used. The detection apparatus is mounted for example in an exhaust pipe of an internal combustion engine with the transition metal oxide exposed to the exhaust gases, and the electric resistance variation thereof is detected. A reference voltage set by reference resistors is applied to the non-inverted input terminal of the amplifier, and a voltage established by the electric resistance of the transition metal oxide is applied to the inverted terminal of the amplifier. The amplifier compares the voltages applied to both its input terminals and produces a corresponding output signal, and the latter signal can be utilized to control the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine.
However, in order to effect proper control of the air-fuel ratio by employing the abovementioned detection apparatus, it is necessary to compensate the electric resistance variation of the transition metal oxide due to temperature variation of the exhaust gases since the electric resistances exhibited by the transition metal oxide vary depending upon not only the concentrations of the gaseous components of the exhaust gases, but also the temperature thereof. For example, in the case where the reference voltage is set to control the air-fuel ratio to the stoichiometrical one at an exhaust gas temperature of 850.degree. C., the control can be preferably effected at this temperature. However, when the exhaust gas is at 350.degree. C., the determined air-fuel ratio is smaller than the stoichiometrical one, which makes precise control of the air-fuel ratio impossible.